An electron beam gun, or emitter, is an electrical component used in a wide variety of vacuum devices. Low power electron beam guns, for example, are commonly used in cathode ray tube (CRT) displays. High power electron beam guns are used in microwave linear beam vacuum devices such as klystrons and gyrotrons, which have applications to particle accelerators and nuclear fusion reactors. For example, the International Thermonuclear Experimental Reactor (ITER) tokamak has an electron cyclotron heating (ECH) system that uses gyrotrons to inject over 20 MW RF power into the plasma. Unfortunately, current gyrotrons lack reproducibility of power and efficiency parameters, most likely due to material variations and variability in the mechanical alignment and precision of the assembly of its components. Velocity spread has been identified as one of the main contributors to low gyrotron efficiency. One of the main sources of velocity spread is the deviation in the geometry and position of the electrodes and cathode. Small variations in the spacing and position of the electrodes can lead to a significant increase in the velocity spread and degradation of the device efficiency.
Current electron beam gun fabrication approaches are based on conventional assembly techniques; these consist of in-process machining, pinned joints and manual alignment by the gun builder. Alignment pins are used to locate precisely positioned bores formed in mating components, along with iterative manual adjustment to align critical features. Typical electron gun assembly techniques also require clearance between parts to allow for assembly, which inherently limits the achievable precision of the fabrication process. Although these fabrication approaches have been successfully employed in the past for many types of electron guns, improved precision alignment and fabrication approaches are needed for high power and high frequency applications because the clearances and tolerances achieved by conventional assembly methods result in detrimental phenomena such as frequency deviation and velocity spread, which reduce the efficiency of a device and similarly diminish the consistency of devices which are produced to satisfy the same specifications. Alignment precision and repeatability at the micron level are needed to reduce such effects in these high power applications.
There is thus a need to develop new technologies which would improve the mechanical alignment of critical gyrotrons components, particularly high power RF electron beam guns.
Villanyi in U.S. Pat. No. 4,607,187 refines the conventional approach for the alignment of electron beam gun components. Oblong and triangular alignment features are formed within the relevant components, and specially configured precision alignment pins leverage the geometric properties of these features to provide alignment of the beam-shaping apertures of the components. While this formulation of a pinned joint technique anticipates gains in the precision associated with the alignment of an individual electron beam gun, a moderate degree of complexity is inherent to the subsequent manual alignment utilized in such an alignment operation, which will lead to variation in the fabrication and performance of identically constructed devices. Moreover, the slight deviations in form and position between the nominal design and the actual alignment features of each of the components will exacerbate the variation of alignment that is associated with this technique, and therefore the increase the discrepancy in performance between devices.
Scarpetti et al. in U.S. Pat. No. 5,416,381 discuss a scheme for aligning the components of an electron beam gun while streamlining the associated assembly process. This methodology is reliant upon the use of ceramic standoffs as alignment features, which provides desirable electrical isolation of the cathode from the anode and thereby reduces the number of necessary components. However, this technique fails to achieve sub-micron precision in the alignment of these critical device components, citing machining tolerances of ±0.0005 in. which are applied to the alignment bores. As discussed previously, this limited precision will result in inefficient operation and will incur poor repeatability and corresponding variation between identical devices.
A kinematic coupling is a device used in a variety of applications requiring the alignment of mating components to be precise and repeatable. In order to fully constrain the respective orientation of two mating components, a kinematic coupling forms deterministic contact between mating elements of each component. In a typical kinematic coupling, there are few contact locations, each constraining one degree of freedom between the mating components. The loading which can be sustained by this approach is fundamentally limited by the Hertzian contact stresses incurred at the point contact regions where the elements meet, rendering kinematic couplings generally unsuitable for use in machining operations and other processes with high loading demands.
In U.S. Pat. No. 6,193,430, Culpepper and Slocumb undertake twofold approaches to the problem of increasing the mechanical loading capacity of a kinematic coupling joint. By implementing a quasi-kinematic coupling, with spherical convex contact surfaces mating with conical concave depressions, the regions of point contact in a true kinematic coupling are replaced by line contact regions, augmenting the load-bearing capacity of the contact in the direction normal to the conical depressions. In addition, the selection of deformable materials to form the convex contact surface will allow the opposing mating surfaces to be brought into contact by the preloading fastening force, allowing the bulk material of the mating parts to bear load normal to these faces. However, the use of deformable materials reduces the precision and repeatability of the coupling while diminishing thermal stability in high temperature applications and rendering such a coupling incompatible with ultrahigh vacuum environments.
In addition, there are several other problems with the use of a kinematic coupling in an electron gun, or even generally in an electron beam device. In an electron gun there are high electric gradients with DC operating voltages of 100 kV or greater. There are also very high thermal gradients with electron beam emitter, the cathode, typically operating at a temperature of 1000 degrees C. or higher. Two additional considerations are that all materials utilized in a gyrotron gun and most other gyrotron gun should be non-magnetic, as well as that the device undergoes thermal processing “bake-out” at 500-600 degrees C. for a period of up to one week. These constraints present significant technical barriers to the use of kinematic coupling in an electron gun.